1. Field of the Invention
The invention herein relates to dot matrix printers and print heads. More particularly it relates to the structure of such print heads and their method of operation.
2. Description of the Prior Art
Dot matrix printers are widely used for transaction receipts and convenience printing. These applications include cash registers, automatic teller machines, gas pumps, lottery tickets, credit card verification, bar code printing, etc. The dot matrix printer, like other impact printers, can produce multiple carbon (or carbonless) copies. It is important for the print head, to be durable, to operate at high speed, to be as simple as possible in construction and to be highly reliable.
Impact print heads operate by forcing the tip of one or more of a plurality of elongated pins against an inked ribbon which is adjacent to the paper or other substrate on which the printed characters are to be formed. The impact of each pin tip drives the ribbon against the paper and leaves an inked dot in the location of that pin. By controlling the number and location of the pins which are activated, and by moving the pin array and paper relative to each other between successive activations of the pins, a collection of such dots is produced which appears as a number, letter, other character, or as a picture, figure or design.
When the print head is in operation, electromagnetic actuators provide force to armatures which act on the driving end of the pins causing them to be reciprocated rapidly to create lines of text, drawings, etc. Ideally the armatures should move only in the back-and-forth direction of print activation. However, in many prior art print heads, armatures also experience substantial lateral motion within their travel channels, which causes substantial wear on the armatures and their mating surfaces at the pivot point, reducing print head life. Commonly print head life has been no more than 100 million characters prior to needing replacement. For high speed dot matrix printers which are used heavily, this translates into relatively frequent replacement of print heads, which is of course uneconomical for the printer user.
To make the most efficient use of high speed dot matrix printers, many users run multi-sheet documents through them, so that with one pass the printer will make multiple copies of each document. In order to maximize the number of layers of paper (i.e., numbers of copies) which can be legibly printed, the pins must be driven firmly against the ribbon and document. In many prior art printers the performance of the print head is not sufficient to produce the number of legible copies per pass that the user wants, and so additional sets of the same document must be run in order to produce the desired total number of copies. If the performance of the print head could be improved, the number of repetitions of a document could be reduced for creation of the same number of copies.
In order to achieve sufficient striking force against the print ribbon to form a clean character on multiple layers of paper, the print wire must be relatively stiff. Since the driving magnets for the print wires are usually arranged in a circle, the print wire must be bent to achieve a vertical column at the output guide. The greater the curvature of the print wire, also related to pullback, the more friction, the more wear and the more driving force is needed. It is the goal of the head design to minimize pullback, lower friction and to reduce wear.
Numerous attempts have been made to alleviate the friction and pullback problems. In most cases, the print wire guides are designed to ease the movement of the print wires through their curves. Such devices have been modestly successful, but problems of friction and wear remain significant.
Most of the print heads with the magnetic gap at the outside pole have armature pivots at the inside pole of the magnetic yoke. While this provides the maximum room for the coils, it limits the lever ratio of the armature which reduces the stroke of the print wire. Using the pivot point at the inside pole also has two other limitations: (1) the print wire length is much shorter than the length of the head, and (2)when the pivot point wears it reduces the stroke of the print wire. The shorter print wire increases the curvature, the stress and the wear between the wire and the guides. An alternate design has the armatures pivoted at the outside pole, then the coils and the active gap are located at the inside pole. In this case, the coils are crowded and the cooling of the coils are less efficient.
Many prior art printers are capable only of printing in the forward direction (i.e., left to right across the paper). This requires the carriage to return to the start of a each line, and the return time is simply lost time from the printing process. Some prior art printers have improved print speed by being capable of linear bidirectional printing: both forward (left to right) and backward (right to left), so that there is no lost carriage return time. A printer that simply prints back and forth requires special logic to reverse the character string and the font, but it requires nothing special from the print head. However, most prior art printers do not have the capability of orthogonal bidirectional printing, i.e., the capability to have the head to move in horizontal and vertical (orthogonal) directions. The orthogonal motion of the print head requires a special output pattern of the print wire to be designed into the print head.